1. Field of the Invention
This relates to power take off assemblies, and saws using power take off assemblies, for example concrete and slab saws and self propelled saws.
2. Related Art
Power saws typically include a motor, engine or other system for producing drive power, a saw blade or other cutting device and a support frame structure for supporting the engine and cutting blade. The power system may be an internal combustion engine, a hydraulic motor, an electric motor or the like. The power system can drive the cutting blade through a mechanical or hydraulic drive system, but a belt drive system is common. The size and dimensions of the saw will vary according to usage and application.
For large projects, higher horsepower levels are desired for the saw. Higher horsepower allows the use of larger cutting blades, reduces cutting time and may provide higher cutting speeds. However, higher horsepower levels usually mean larger engines and often larger saw dimensions. Larger saws may also mean less maneuver ability.
In some applications, structures may be such that access to an area for cutting may be limited. For example, in high-rise buildings, concrete floors and/or walls may be poured or installed on an ongoing basis, and detail work may come later. Access openings such as door ways, floor openings, and the like, as well as concrete openings for fixtures may be formed after concrete slabs and walls are poured and the concrete hardened. Similar work may also be done on other concrete projects after the concrete has hardened. However, the equipment to be used for cutting in those areas may be limited by such restrictions as access opening size, elevator size and the like. For example, door ways in buildings may be 32 inches in width, thereby limiting the width of the saw to less than 32 inches. Therefore, the amount of horsepower for a saw and other operating characteristics such as blade size and the like may be limited for a given application by such factors as access opening size and the like.
Increasing the horsepower for a saw often results in a larger saw. The consequences of a larger saw may be most noticeable where the saw engine is oriented sideways relative to the forward direction of motion of the saw. In these saws, the engine crank shaft and the stub shaft attached to it extend sideways from the engine, and one or more drive belts couple the stub shaft sheave to a sheave for driving a saw blade shaft. If the engine is made larger, the width of the engine with the stub shaft typically increases, which may make the saw less maneuverable. Therefore, saw improvements may have undesirable consequences for some users.
In saws with a belt drive for the saw blade, the sheave is typically mounted to and held on the stub shaft using a shaft key fitting into a longitudinal groove in the stub shaft and a corresponding groove in an internal surface of the sheave. The stub shaft, key and sheave are subjected to side loading by the belts during normal operation. The side loading and the rotation in turn produces a cyclical fatigue load on the stub shaft, often focused in the area of the shaft key. Such loads sometimes produce fatigue and sometimes fractures in the shaft, leading to drive failure. Increasing the saw horsepower produces more cyclical fatigue loads on the shaft and may increase the likelihood of shaft failure.